Thanks for looking into this Mike.
Nephele was shaped by Zeus from a cloud formation (= "Nephele" in Greek)in the form of his wife Hera. The Centaurs were indeed her offspring with Ixion. Nephele eventually married king Athamas and gave birth to Phrixus and Helle. Some mythological accounts recount that these two different Nepheles, although the consensus is that we are talking about the same person.
A brief (and certainly not definitive) account of Nephele is given by:http://www.geocities.com/Athens/Oracle/5545/beauties-nephele.html